Add option to add new projects for Splunk

[martinvisser]

Currently it's only possible to add projects to the Splunk plugin's configuration by hand. There is also no link between an index and a project.

{
  "base_url" : "https://where.splunk.lives",
  "app" : "app",
  "indexes" : "index1, index2",
  "search_field" : "app-name",
  "projects" : [ {
    "name" : "project-1"
  }, {
    "name" : "project-2"
  } ],
  "plugin_version" : "0.2.0"
}